Risk factors for cancer from the ambient environment are studied to identify specific chemicals and classes of contaminants, to investigate mechanisms of action, and to estimate the contribution of environmental factors to cancer in the general population. Case-control studies are conducted on non-Hodgkins lymphoma and cancers of the lung, bladder, colon, rectum, stomach, esophagus, brain, pancreas, and kidney. Exposures include drinking water contaminants, especially disinfection byproducts, nitrate, and arsenic; airborne radon in homes; and body burdens of chlorinated hydrocarbons from past environmental or dietary exposures. Related case-control studies in Iowa showed excess risks for rectal cancer in both sexes and bladder and brain cancers among men after long-term consumption of disinfection byproducts in drinking water. A Mexican study, in which NCI researchers were major collaborators, did not find a link between breast cancer risk and serum DDT levels. Studies in Taiwan and elsewhere have described a high risk for bladder and other cancers after exposure to arsenic in drinking water supplies at levels several times the maximum contaminant limit. A case-control study in Utah is evaluating bladder risk at lower levels of arsenic that are more common in the U.S. Stimulated by findings from an earlier NCI study that found a link of non-Hodgkins lymphoma with drinking water nitrate, the nitrate in drinking water was evaluated in a case-control study in Minnesota, with negative findings. An ongoing case- control investigation in Iowa is also evaluating this hypothesis. The issue of nitrate as a cancer risk factor is being pursued further in an ecologic study encompassing eleven states in the United States. A case-control study of lung cancer and residential radon among Missouri women is unique because it used a novel radon detector which integrates residential radon exposure over the past 30 years (CR-39 detector). A significant excess lung cancer risk was observed with increasing radon concentrations when measure by the CR-39 detector but not when measured by standard radon dosimetry. The CR-39 detector is undergoing further field testing in Minnesota and Helsinki, Finland. Several activities are developing new approaches, and improving existing methods, of exposure assessment in studies of general environmental exposures. These are required to better estimate risk and to detect the relatively small increases in risk often encountered in such studies. Geographic information systems (GIS) are being explored for their utility in environmental epidemiology studies for locating populations potentially exposed to agricultural pesticides and for classifying populations with respect to their water source. Data bases of water contaminants, gathered for routine monitoring purposes, are being used to estimate past exposures to subjects in case- control studies. - Human Subjects & Human Tissues, Fluids, Cells, etc.